The number of cars using the Channel tunnel went up 30 per cent compared with July, spurring Eurotunnel into a war of words with its largest competitor, P&O European Ferries.

Eurotunnel claimed that it had become the market leader on the Dover/Folkestone- Calais route but last night this appeared to have been refuted by the ferry operator.

Bill Mackenzie, Eurotunnel's acting commercial director, said the figure of 143,577 cars going through the tunnel in August represented a 39 per cent share of the cross-Channel car market on this route.

However, P&O had issued figures on Friday showing that it had taken 176,590 cars last month on that route. Even without including coaches, caravans, campers and trailers, all excluded by Eurotunnel as they have not fully developed these services, P&O appears to have considerably outperformed Eurotunnel.

P&O's spokesman, Ian Todd, said: "Eurotunnel's figures for our performance are inaccurate because they obtained them by standing on the cliffs at Dover counting cars. That's a ridiculous way of going about it."

He said that counting all vehicles, Eurotunnel obtained just under 30 per cent share of the tourist market, while P&O had about 37 per cent. The other two ferry operators, Stena and Snat, shared about 26 per cent, with Hoverspeed obtaining the rest.

Overall, Mr Todd estimated the August market had grown about 15 per cent and P&O had shown a 9 per cent fall on last year's August figure because of competition from the tunnel.

The war of words overshadowed Eurotunnel's genuinely impressive performance. However, until next month when it issues financial figures for the summer quarter, the extent to which it has bought its market share through discounting will not be known.

On freight, even Eurotunnel's competitors admitted that its figure of 36,517, a very small dip of 1.6 per cent on July, was a strong performance in a traditionally poor month. Eurotunnel said it had achieved a 49 per cent share of the Dover/Folke- stone-Calais lorry market.

The fierce competition for cross-Channel traffic was increased still further yesterday when ferry operator Stena Sealink announced expansion plans that would create 600 jobs.

The company said from next year it was boosting its three-strong Dover- Calais fleet with a new 2,300-passenger superferry. The 28,727-tonne Stena Jutlandica, would be the biggest ship ever to sail the Straits of Dover.

Channel tunnel services have also hit the airlines, with an estimated 13 per cent decline in the number of passengers flying routes from Heathrow to Paris and Brussels in the first six months of 1995. The figure comes from British Midland, the UK's second-biggest operator, which said it did not expect airlines to feel the full impact of rail services until next year.

Although the company said its own share of these routes fell only1.6 per cent in the first half, Austin Reid, managing director, said: "I do not think you have to be a detective to realise that this means somebody, somewhere, is hurting."

Air Liberte, the French airline, announced on Monday that it was halting flights between Paris's Orly airport and Gatwick because competition from Eurostar made the service uneconomic.

Yesterday, British Midland announced plans to open a service from Heathrow to Zurich from 29 October. It said it would match British Airways' four services a day on the route, one service less than Swissair.